my strong recommendation is to keep your epoxy inside and at a normal room temperature or better.

+1 on that.
You've put a lot of work and money into the project already, and an epoxy disaster is not what you need.

Is your shed open-sided or enclosed? A few hundred dollars in insulation and plastic sheeting, and some electric heaters could raise the working temperature high enough to do the job more easily.

Or, tape the hull and deck together to keep them 'stable (they are glassed on both sides?) and work on something else until spring.....